In a fabrication process for a semiconductor element such as an LSI or VLSI formed with an ultra-micropattern, a reduction projection exposure apparatus has been used which prints a circuit pattern formed on a master such as a reticle onto a substrate coated with a photosensitive agent (resist) by reducing and projecting the circuit pattern. Conventionally, further micropatterning is required to increase the packing density of circuit elements. With this background, a resist process has been developed and a projection exposure apparatus has been improved to cope with micropatterning.
Means for improving the projection exposure apparatus to cope with micropatterning includes improvement in alignment precision with which the pattern is correctly overlaid. Factors that largely influence the alignment precision include a magnification error or distortion error of a projection optical system.
A magnification error or distortion error appears as a difference between a desired grid point and the grid point of a projection pattern. Such magnification and distortion errors of the projection optical system are corrected by adjustment in the manufacturing process of the projection optical system and adjustment of the exposure apparatus in installation. These errors are known to change in accordance with the ambient atmosphere, particularly the atmospheric pressure and temperature. The projection optical system absorbs an exposure energy during wafer exposure. This changes optical elements (e.g., a refractive index and shape), thus changing any magnification or distortion errors.
As a method of correcting magnification and distortion errors caused by a change in atmospheric pressure or temperature, light absorption, and the like, the present applicant has proposed a method of arranging a plurality of correction lenses, which are movable in the direction of an optical axis, in a projection optical system, and changing the positions of the correction lenses, and a method of changing the wavelength of exposure light (Japanese Patent No. 2,897,345).
While measures for micropatterning have been sought, a further increase in throughput of the exposure apparatus has been attempted to reduce the fabrication cost of a semiconductor element. For example, an exposure time per shot may be shortened by increasing the output of the exposure light source. Also, the number of elements per shot may be increased by enlarging the exposure area.
In recent years, in order to cope with an increase in chip size of the semiconductor element, a shift is taking place from a step & repeat, so-called stepper, which sequentially prints a mask pattern while moving the wafer step by step, to a step & scan exposure apparatus, which scans and exposes while synchronizing the mask and wafer to each other and with which exposure sequentially moves on to the next shot when exposure of one shot is ended.
The step & scan exposure apparatus has a characteristic feature in that, as its exposure field has a slit-like shape, its exposure area can be enlarged without increasing the size of the projection optical system.
According to the step & scan exposure apparatus described above, when step movement is to be performed before exposure of each shot region in order to print a mask pattern onto a corresponding shot region on the wafer, the correction lens of the projection optical system is moved in the direction of the optical axis in accordance with a change amount of the atmospheric pressure, temperature, and light absorption, so magnification and distortion errors of the projection optical system are corrected. At this time, when the difference between the target position and current position of the correction lens falls within a predetermined range for a predetermined period of time or more, it is determined that positioning is complete, and exposure is performed immediately after this.
As the NA of the projection optical system increases, the output of the light source increases, or a switching mechanism for performing various types of illumination modes is loaded in accordance with employment of a modified illumination method that achieves high resolution by performing illumination while controlling the distribution of secondary light sources in various manners, the illumination optical system tends to increase in size and weight. This degrades the anti-vibration characteristics of the exposure apparatus.
Degradation in anti-vibration characteristics during exposure is apparent particularly when the step & scan exposure apparatus described above which performs scanning and exposure is employed. Assume that exposure is performed with a positional displacement of the correction lens of the projection optical system being caused by the operation of other portions of the exposure apparatus during exposure, or by a disturbance such as an external vibration, that is, in the presence of a projection magnification error and distortion error. Then, a device that does not have a necessary alignment precision is undesirably forwarded to the next step (developing step or the like).
So far, a description has been made of the magnification and distortion errors of the projection optical system. Other than these errors, for example, a focus error or curvature of field of the projection optical system is preferably considered.